A reliable system for the growth and quantitation of erythroid cells obtained from human peripheral blood and bone marrow and murine spleen and bone marrow has been established. This culture system has been employed in three areas. First, it provides means to follow patients with immune suppression of bone marrow on treatment, including plasmapheresis and chemotherapy (See clinical protocol 79-H-90). Second, erythroid cell culture has served as the basis for an assay system to detect antibodies which may functionally inhibit erythropoietin activity (see project #Z01 HL 02302-02 CHB). Finally, the cell culture system is of intrinsic interest because of its relevance to questions concerning the regulation of globin gene expression.